1. Field of the Invention
The present invention relates to a motor drive device which has the function of detecting contact fusion in an electromagnetic contactor by utilizing a function originally provided in the motor drive device.
2. Description of the Related Art
The motor drive device which drives a machine tool or industrial machine, robot, etc. receives as input power a three-phase alternating current. This is converted at an AC/DC converter to a direct current voltage. The converted direct current voltage is converted at a DC/AC converter to a desired frequency of alternating current to drive the motor. In such a motor drive device, an electromagnetic contactor is set at an input power line to the AC/DC converter.
An electromagnetic contactor has a built-in contact. When the electromagnetic contactor is turned on, the contact is closed to make the input side and output side of the electromagnetic contactor conductive, while when the electromagnetic contactor is turned off, the contact is opened to break the conduction of the input side and output side of the electromagnetic contactor. In the electromagnetic contactor which is set at the input power line, at the time of an emergency stop or alarm, the electromagnetic contactor is turned off so as to open the contact and separate the motor drive device from the power supply. If the contact of the electromagnetic contactor is opened, the supply of energy from the power supply is stopped and the motor stops operating.
However, sometimes the load of the motor becomes large and an excessive current flows to the electromagnetic contactor and sometimes long years of use of the motor drive device causes the contact of the electromagnetic contactor to fuse. If the contact of the electromagnetic contactor fuses, even if the electromagnetic contactor is turned off, the contact will not open, current to the motor will not be able to be cut off, and unexpected operation of the motor may cause a dangerous situation.
As a countermeasure to this, for the electromagnetic contactor which is set at the input power line of the motor drive device, one which is provided with an auxiliary contact is used. The auxiliary contact is a contact which operates linked with the main contact of the electromagnetic contactor and is used to detect fusion of the main contact. Further, when detecting fusion of the main contact of the electromagnetic contactor, the method is generally adopted of stopping the motor driving the machine, turning the electromagnetic contactor on/off in the state where the motor is not consuming energy, and monitoring the opened/closed state of the auxiliary contact of the electromagnetic contactor at this time. Such an auxiliary contact of an electromagnetic contactor is, for example, disclosed in the product information of the site of Hitachi Industrial Equipment Systems Co., Ltd., Standard Type Electromagnetic Contactors, Switches [HS Series] (http://www.hitachi-ies.co. jp/products/hdn/mgsw/).
However, in such a method of detection of fusion of a contact in an electromagnetic contactor, an electromagnetic contactor with an auxiliary contact is necessary and an additional circuit for monitoring the auxiliary contact is necessary inside or outside the motor drive device, so there is the issue that this leads to an increase in costs.